The invention generally relates to 4-terminal driver for a substantially capacitive load. In particular, the driven can be used for driving switching power devices having high input capacitance, such as MOSFETs. The driver is characterized by a very high switching speed and is intended to operate in systems requiring a constant or occasional switching, e.g. switching power supplies and switching power amplifiers. The driver can be used as buffer in a driver circuit with floating or grounded bias supply.
The gate impedance of a power MOSFET is a high resistance shunted by a large composite gate capacitance, including Miller effect capacitance. Therefore, the MOSFET requires fairly large input current to change the gate voltage. Due to a limited transconductance, the switching speed of the MOSFET depends upon the rate at which gate charge is supplied or removed. Consequently, the MOSFET driver should minimize any lead inductance by reducing loop areas. For instance, each inch of wire adds about 20 nH of inductance.
The conventional drivers comprise complex current pulse drive circuits. The employment of transformers having a small turns ratio results in large currents which must be delivered to the primary winding. Moreover, numerous problems are introduced. For instance, transformers are bulky and expensive. Their high frequency performance is very poor. Snubber and clamp circuits are necessary to prevent forward and reverse voltage spikes, and reduce RFI. Some other methods significantly improve the performance by employing a level shifter. One technique is bootstrapping, another one is disclosed in the abovementioned "Power Switch Driver" U.S. Pat. No. 4,940,906 dated Jul. 10, 1990, by the same inventor. However, even these techniques do not address the problem of the buffer which explicitly drives capacitive load, e.g. MOSFET. The buffer usually comprises a push-pull output stage with resistors limiting output currents. The level shifter must deliver energy which is wasted in the buffer.